[meteorite-list] Magnetic fields of tetrataenite particles in pallasites shed light on earth's magnetic core

[J Sinclair]

That's an excellent article for better a understanding of the
pallasites plus reference to pallasites we all know - Esquel, Imilac
and Brenham.

Thanks!

John

Here is the editor's summary from Nature.

Shortly after the birth of the Solar System, small planetary bodies
became hot enough to segregate into a liquid metal core surrounded by
rocky mantle. As the core cooled and froze, swirling motions of liquid
metal, driven by the expulsion of sulphur from the growing inner core,
generated a magnetic field. A class of meteorites known as pallasites
preserves this phase of Solar System history as in the form of
gem-quality crystals of the silicate mineral olivine embedded in a
metallic matrix of iron–nickel alloy. James Bryson et al. use
high-resolution magnetic imaging of the iron–nickel matrix of the
Imilac and Esquel pallasite meteorites to derive a time-series record
of magnetic activity on the pallasite parent body, encoded within
nanoscale intergrowths of iron-rich and nickel-rich phases. This
record captures the dying moments of the magnetic field generated as
the liquid core solidified, providing evidence for a long-lasting
magnetic dynamo driven by compositional convection.

On Wed, Jan 21, 2015 at 8:26 PM, Robin Whittle via Meteorite-list
<meteorite-list at meteoritecentral.com> wrote:
> Here is a write-up of some interesting research.
>
>   - Robin
>
>
>   http://phys.org/news/2015-01-death-dynamo-hard-space.html
>
>      The researchers' magnetic measurements, supported by computer
>      simulations, demonstrate that the magnetic fields of these
>      asteroids were created by compositional, rather than thermal,
>      convection - meaning that the field was long-lasting, intense and
>      widespread. The results change our perspective on the way magnetic
>      fields were generated during the early life of the solar system.
>
>      These meteorites came from asteroids formed in the first few
>      million years after the formation of the Solar System. At that
>      time, planetary bodies were heated by radioactive decay to
>      temperatures hot enough to cause them to melt and segregate into a
>      liquid metal core surrounded by a rocky mantle. As their cores
>      cooled and began to freeze, the swirling motions of liquid metal,
>      driven by the expulsion of sulphur from the growing inner core,
>      generated a magnetic field, just as the Earth does today.
>
>      "It's funny that we study other bodies in order to learn more
>      about the Earth," said Bryson. "Since asteroids are much smaller
>      than the Earth, they cooled much more quickly, so these processes
>      occur on shorter timescales, enabling us to study the whole
>      process of core solidification."
>
>      Scientists now think that the Earth's core only began to freeze
>      relatively recently in geological terms, maybe less than a
>      billion years ago. How this freezing has affected the Earth's
>      magnetic field is not known. "In our meteorites we've been able to
>      capture both the beginning and the end of core freezing, which
>      will help us understand how these processes affected the Earth in
>      the past and provide a possible glimpse of what might happen in
>      the future," said Harrison.
>
>      However, the Earth's core is freezing rather slowly. The solid
>      inner core is getting bigger, and eventually the liquid outer core
>      will disappear, killing the Earth's magnetic field, which protects
>      us from the Sun's radiation. "There's no need to panic just yet,
>      however," said Harrison. "The core won't completely freeze for
>      billions of years, and chances are, the Sun will get us first."
>
> The article itself is behind a paywall:
>
>   http://www.nature.com/nature/journal/v517/n7535/full/nature14114.html
>
>   Long-lived magnetism from solidification-driven convection on the
>   pallasite parent body
>
>     James F. J. Bryson et al.
>     Nature 517, 472–475 (22 January 2015)
>     doi:10.1038/nature14114
>
>      Palaeomagnetic measurements of meteorites suggest that,
>      shortly after the birth of the Solar System, the molten
>      metallic cores of many small planetary bodies convected
>      vigorously and were capable of generating magnetic fields.
>      Convection on these bodies is currently thought to have
>      been thermally driven, implying that magnetic activity
>      would have been short-lived. Here we report a
>      time-series palaeomagnetic record derived from nanomagnetic
>      imaging of the Imilac and Esquel pallasite meteorites, a
>      group of meteorites consisting of centimetre-sized metallic
>      and silicate phases. We find a history of long-lived magnetic
>      activity on the pallasite parent body, capturing the decay
>      and eventual shutdown of the magnetic field as core
>      solidification completed. We demonstrate that magnetic
>      activity driven by progressive solidification of an inner
>      core, is consistent with our measured magnetic field
>      characteristics and cooling rates. Solidification-driven
>      convection was probably common among small body cores, and,
>      in contrast to thermally driven convection, will have led
>      to a relatively late (hundreds of millions of years after
>      accretion), long-lasting, intense and widespread epoch of
>      magnetic activity among these bodies in the early Solar
>      System.
>
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